Mercury is the first planet from the Sun. It is also the least massive and the smallest, being slightly smaller than Jupiter's moon Ganymede. Mercury orbits the Sun in 88 Earth days, which is the shortest orbit in the Solar System. The surface temperature on the sunny side of the planet is around 400 degrees Celsius, while the dark side experiences temperatures below 170 degrees Celsius. This huge difference is possible because Mercury does not have a significant atmosphere, so there is no medium to transfer heat from the light to the dark side (unlike Earth. The absence of an atmosphere means there is no weather to speak of, and the surface of the planet is pockmarked with impact craters. Mercury has no natural satellites. The first space probe to visit the planet was the American Mariner 10 in 1974. Later, NASA launched MESSENGER, the second probe to reach the planet. Mercury has a large iron core, taking up about 80% of its volume. It is about six times less massive than the Earth.